1. Field of the Invention
The present invention relates to a balanced battery cell charging circuit. More particularly, the present invention relates to a battery charging circuit that monitors the voltage of each cell within the battery, and supplies a charging current depending on the cell voltage. Particular utility of the present invention is a battery charging circuit for portable electronic devices; although the present invention has utility in any system that uses rechargeable batteries.
2. Description of Related Art
Various charger circuits and techniques for charging and recharging secondary cells are known. In one such technique, the cell voltage is monitored and a charge current supplied to the cell is reduced as the cell voltage increases. This technique is based on a recognition that, as the voltage across the cell increases, its charge acceptance decreases. Other battery charging techniques utilize circuitry for sensing the charge accepted by the cell and reducing the charge current supplied to the cell as the accepted charge decreases. In still another battery charging technique, a constant current is supplied to the cell during a first charging interval and a constant voltage is provided to the cell during a second charging interval. The first and second intervals may have predetermined durations or alternatively, may be a function of a battery condition, such as the cell voltage.
As is apparent, many battery charging techniques require measurement of the voltage across the rechargeable cell. Another reason for measuring the cell voltage is to prevent cell damage due to an overvoltage or undervoltage condition. More particularly, certain types of nonaqueous electrolyte battery cells, such as lithium ion cells, are susceptible to damage if charged to too high a voltage or permitted to be discharged to too low a voltage.
Secondary cells are often connected in series to power a load, since the total voltage across the string of series-connected cells is approximately equal to the sum of the voltages across each individual cell. One way to measure the individual cell voltages in a string of series-connected cells is to measure the total voltage across the string of cells and divide the measured voltage by the number of cells. However, this technique provides only a rough approximation of the individual cell voltage since typically, the voltage across each cell varies somewhat.
Another technique for measuring the voltage across individual series-connected cells is to provide a sensing circuit for each such cell and average the outputs of the sensing circuits. For example, a plurality of differential amplifiers may be provided, with input terminals of each amplifier coupled across a respective cell and the output signals of the amplifiers averaged. However, since such a measurement is of the average cell voltage, when using the measurement to control cell charging, some cells will be overcharged and others will be undercharged in accordance with the deviation between their respective voltage and the average measured voltage. Moreover, use of plural sensing circuits results in disadvantageous component duplication and concomitant increases in manufacturing time and cost.
One attempt to solve these attendant problems can be found in U.S. Pat. No, 5,652,501. This patent discloses battery charger/monitor circuit for charging and/or monitoring a plurality of series-connected cells. The disclosed circuit includes a voltage sensor for sensing the voltage across each of the cells to provide a high cell voltage signal proportional to the highest voltage across any of the cells and a low cell voltage signal proportional to the lowest voltage across any of the cells. The circuit is operable in a monitor mode or a charge mode. In the monitor mode, the cells are disconnected from a load if the low cell voltage signal decreases to a first predetermined level. The circuit also includes a controller that provides a control signal in response to the high cell voltage signal, the low cell voltage signal and a current sense signal, for controlling the charging of the cells. In the charge mode, the cells receive a constant charge current until the high cell voltage signal reaches a second predetermined level, after which the voltage across the cell charged to the highest voltage is held substantially constant, causing the charge current to be reduced.
While this alleviates some of the attendant problems associated with the prior art, this attempt does not provide a circuit that considers power dissipation criteria. For IC implementation, there is often a limit as to the maximum power that the IC is permitted to dissipate. Also, for portable device applications, it is necessary to be very power conscious, for obvious reasons. In the aforementioned patent, the disclosed topology reduces the cell voltage once a predetermined threshold is met. However, this cannot accurately monitor power dissipation considerations, nor can charging current be adjusted at a battery cell level.